Films of certain polymers are capable of acquiring a static electric charge upon being poled, i.e., being exposed to a corona discharge or a dc field, which static charge is quasi-permanent, the degree of permanency being dependent largely upon the chemical constitution of the film. Such a charge, due largely to trapped positive and negative carriers positioned at or near the surfaces of the film, produces what is known as an electret. The term "film", as used herein, is understood to include thicknesses up to about 0.060 inches.
In addition to being capable of acquiring a static electric charge, the polymer film may also acquire an ordered internal molecular orientation when electrically polarized so that even when the static charges are dissipated, the film thereafter possesses the property of being able to generate an electric current between opposed surfaces thereof, when electrically connected, by applying a load or pressure to the opposed surfaces. This property is known as the piezoelectric effect. Films made from poly(vinylidene fluoride) (PVDF) or vinylidene fluoride polymers, for example, may be poled to exhibit this effect as well as a pyroelectric effect, i.e., the property of producing electron flow by changing the temperature of the film.
The object of poling is to align the dipoles within the polymer film such that the film displays permanent piezo and pyroelectric properties. The dipolar alignment in a piezoelectric polymer film is in an opposite direction to the polarizing field while charges injected into the film are of the same sign as the polarizing voltage. Charges on the film surface of opposite sign to the polarizing voltage imposed on that surface are termed "heterocharges" while charges of the same sign are termed "homocharges". Heterocharges and homocharges are described and shown in U.S. Pat. No. 4,055,878, incorporated herein by reference.
The extent to which the dipoles of the polymer film have been aligned can be determined by several methods. A simple procedure involves the application of a load to the film in a direction normal to the film's surface and detecting the resultant charge output on a suitable meter. If the resultant charge on the surface of the film is a heterocharge, the output charge due to the imposed testing will be in one expected direction. Conversely, if the resultant charge on the surface of the film is a homocharge, the output charge due to the load will be in the other direction.
The procedure for poling is well known in the art and, in the case of dielectric polymer films, generally involves the application of a direct current voltage, e.g., 300 to 2000 kilovolts per centimeter of thickness of polymer film while first heating it to a temperature ranging between just above room temperature to just below the melting point of the film for a period of time and then, while maintaining the potential, cooling the film. Preferred systems for the continuous poling of piezoelectric (or pyroelectric) sensitive polymer film using a corona discharge to induce the piezoelectric charge are described in U.S. Pat. No. 4,392,178 and U.S. Pat. No. 4,365,283, both incorporated herein by reference.
Poling results in permanent polarization, as abovementioned, due to aligned dipoles. Additionally, real charges or homocharges are injected and concentrated along the film surface, and are of the same polarity as the poling electrode. These homocharges can mask the effect of the permanent heterocharges due to the permanent orientation of the dipoles and can thus produce a false piezoelectricity. Homocharges are capable of remaining on a poled dielectric film or sheet for months unless intentionally removed. Storage of real charges can therefore be quite permanent with long decay times. A further complication is the instability of the heterocharges which can also exist in the polarized polymer films. The present invention is directed toward the removal of both unstable homo- and heterocharges.
Various techniques for stabilizing electrically charged materials or polymer films are disclosed in the prior art. U.S. Pat. No. 3,660,736 discloses a heat treatment for stabilizing electrets having unstable heterocharges and homocharges thereon; U.S. Pat. No. 3,793,715 discloses stabilizing an electret having heterocharges and homocharges by contacting the electret with water to cause decay of the unstable charges; U.S. Pat. No. 3,755,043 teaches the stabilizing of an electret by covering opposite surfaces of a high molecular weight based material before polarization thereof with thin films of another high molecular weight material; U.S. Pat. No. 4,055,878, mentioned above, discloses stabilizing piezoelectric resin elements by applying a high pressure to the poled piezoelectric resin element for a given time, and short circuiting the surface of said element under pressure; U.S. Pat. No. 4,393,093 teaches the enhancement of polarizability of polymer films by treating the film with a glow discharge at a reduced pressure; U.S. Pat. No. 4,459,634 reveals methods for stabilizing a poled polymeric piezoelectric film by exposing it to an electric field of a polarity reversed from that employed in the poling step to remove unstable homo- and hetero-charges from the poled film.
After the vast majority of poling procedures, unstable hetero- and/or homocharges are present in the polymer film; these unstable charges must be removed prior to either using the film in a piezo or pyro device, or performing accurate tests thereon.
The present invention discloses methods for removing these unwanted unstable charges, while virtually leaving undisturbed the important stable aligned dipoles, and is directed to methods of stabilizing a poled piezoelectric or pyroelectric polymer film comprising exposing the surfaces of said poled polymer film to a glow discharge at a reduced pressure for a time sufficient to provide stabilized poled polymer film. The glow discharge bathes the poled film in a medium of gaseous charges which effectively stabilize the film. Treatment of the film in this manner may be adapted for non-continuous batch applications, or for continuous poling operations, as from reel to reel, of poled piezoelectric polymer film. Long delays between the steps of polarization and stabilization can be tolerated in the practice of the present invention.